Julian day

Remarks

Julian days begin at noon, not at midnight, so a night of observations has a single Julian day number. Example: the instant 1 January 2000, 12:00, has Julian date $2\,451\,545.0$; this is the standard epoch J2000.0, in the time scale TT. Example: 1 January 1970, 0:00 UT, has Julian date $2\,440\,587.5$; the fraction $.5$ is half a day after the preceding noon. Non-example: a date such as 1 January 2000 in the Gregorian calendar is not a Julian date; it is a calendar date that must be converted. The modified Julian date is $\mathrm{MJD} = \mathrm{JD} - 2\,400\,000.5$; it starts at midnight. The Julian day has nothing to do with the Julian calendar except that the epoch is expressed in it.